1. Field of the Invention
This invention relates to a powertrain for motor vehicles, in particular to a powertrain having a multiple speed transmission and a transfer case for directing power to front wheels and rear wheels.
2. Description of the Prior Art
A transfer case usually includes a planetary gear set for producing either a “high” range, in which the transfer case output is driven at the same speed as the input, or a “low” range, in which the output is driven slower than its input speed. The 4×2 and 4×4 states of the transfer case are usually selected manually by the vehicle operator by operating a selector, such as a lever or switch. A first position of the selector causes a range selection device in the transfer case to direct power from the transmission output to a rear drive axle, 4×2 drive operation. A second position of the selector causes the transfer case to direct power to both the front drive axle and a rear drive axle, 4×4 drive operation.
Conventionally the high and low ranges are produced by alternately engaging and disengaging a hydraulically actuated range coupler, such as a clutch or synchronizer. When 4×4 drive operation is selected, a hydraulic clutch is engaged; when 4×2 drive operation is selected, the clutch is disengaged. The clutch that controls the 4×4 and 4×2 drive operation typically includes a clutch pack of alternating spacer plates and friction discs, which are forced into mutual friction contact when a piston located in a cylinder is pressurized with hydraulic fluid, thereby engaging the clutch. The clutch is disengaged by venting the cylinder, which allows a spring to release the piston allowing the plates and discs to separate.
A power transmission, which may be controlled manually or automatically to produce multiple speed ratios of the speed of the engine and the speed of its output, is driveably connected to the input of the transfer case, which contains a gearset that produces alternately a high speed ratio range and a low speed ratio range.
There is a need to coordinate control of the transmission and of the transfer case such that the powertrain can increase the available span of speed ratios produced by the transmission alone, and provide selectable driving modes, for which the speed ratios, steps and span correspond to an ideal for the selected operating mode.